> But WebM could be ours instead of theirs, no?
No. WebM could be Google's instead of MPEG-LA's. WebM is a Google project, Google-owned and Google-managed. Much like Android, it is open source so that anyone can see the code and make their own changes, but again, like Android, it is not a community effort. There's a chance that your changes, if useful, will make it back in, but in the end WebM, like Android, is controlled not by the community, but by a corporation.
Essentially the comparison being made is between open source and open standards. The WebM implementation is open-source, but it is not an open standard. There are also open-source implementations of H.264 (e.g. x264), but H.264 is also an ISO/ITU standard which anyone can read and implement. WebM, however, is royalty-free, whereas H.264 is not.
WebM has two of the three (open source, royalty free). H.264 also has two of the three (open source, open standard). If Google submits WebM to a standards body for approval, it will have all three.
Until Google does submit it as a standard, it's still Google's format, not ours, they're just letting us use it for free. Down the road they can always change the format, the container, add new features (like some kind of inline advertisement), and anyone who's invested in implementing this format will have to scramble to add the new functionality or be left behind.
Will they? Who knows, but the price you pay for implementing WebM right now is uncertainty. With H.264, you know exactly where you stand, and the licensing terms are specific, and corporations like that about it.